1. Field of the Invention
The present invention relates to a heat-sensitive imaging element including an IR dye. More particularly, the present invention relates to a heat-sensitive lithographic printing plate precursor including the IR dye. The present invention also relates to a method for making a lithographic printing plate whereby a print-out image of high contrast is formed upon exposure to IR-radiation or heat.
2. Description of the Related Art
Lithographic printing typically involves the use of a so-called printing master such as a printing plate which is mounted on a cylinder of a rotary printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to the image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e., ink-accepting, water-repelling) areas and hydrophilic (or oleophobic, i.e., water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas, and during driographic printing only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called a plate precursor. A typical positive-working plate precursor includes a hydrophilic support and an oleophilic coating which is not readily soluble in an aqueous alkaline developer in the non-exposed state and becomes soluble in the developer after exposure to radiation. In addition to the well known photosensitive imaging materials which are suitable for UV contact exposure through a film mask (the so-called pre-sensitized plates), heat-sensitive printing plate precursors have also become very popular. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method (CtP) wherein the plate precursor is directly exposed, i.e., without the use of a film mask. The material is exposed to heat or to infrared radiation and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer, or by particle coagulation of a thermoplastic polymer latex, and solubilization by the destruction of intermolecular interactions or by increasing the penetrability of a development barrier layer.
It is important in the printing plate preparation work that the exposed plate precursor shows a visible image even before being developed, if necessary, i.e., a print-out image. This enables the end-user to establish immediately whether or not the precursor is already exposed to light, to inspect images on the printing plate and to distinguish the plate as to which color inks should be applied. In such a work flow, the exposed printing plate is developed later in a separate developing step or in an on-press processing step, or is further used in the printing process without the need of a developing step.
On-press processing is disclosed in EP 770494, wherein the plate is mounted on the press and the coating layer is developed by interaction with the fountain solution and ink that are supplied to the cylinder during the press run. During the first runs of the press, the non-exposed areas (for a negative-working precursor) are removed from the support and thereby define the non-printing areas of the plate. Since development of the plate is not carried out before starting the printing process, a previous inspection and discrimination of the plate is not possible unless there is formed a print-out image.
Several methods for forming a print-out image are known for photopolymer systems such as disclosed in U.S. Pat. No. 3,359,109; U.S. Pat. No. 3,042,515; U.S. Pat. No. 4,258,123; U.S. Pat. No. 4,139,390; U.S. Pat. No. 5,141,839; U.S. Pat. No. 5,141,842; U.S. Pat. No. 4,232,106; U.S. Pat. No. 4,425,424; U.S. Pat. No. 5,030,548; U.S. Pat. No. 4,598,036; EP 434968; WO 96/35143; and U.S. 2003/0068575. In these materials, the photoinitiating system is a reacting component which induces formation of the print-out image upon exposure and therefore the performance of the lithographic differentiation process is reduced.
The formation of a print-out image is also known for heat-sensitive lithographic printing plates. The plates are usually image-wise exposed by an IR-laser and consequently are sensitive to IR-radiation. These printing plate precursors include, beside an IR dye as the light-to-heat conversion compound, a dye which absorbs in the visible light wavelength range and which undergoes a color change upon heating. This color change can be obtained with a heat-decomposable dye which is bleached upon heating such as disclosed in DD 213530, EP 897134, EP 925916, WO 96/35143, and EP 1300241.
The color change can also be the result of a shift of the absorption maximum of the visible dye by heating as disclosed in EP 1502736 and EP 419095.
EP 1508440 discloses a lithographic printing process wherein a printing plate precursor includes an IR-dye and a dye-precursor, the dye-precursor having no substantial absorption in the visible light wavelength range. Upon image-wise exposure with IR-light, a dye is formed from the dye-precursor, the dye having an absorption in the visible light wavelength range. EP 1428676 describes print-out formation, upon IR-light exposure, with dye-precursors (i.e., coloration) or with dyes that undergo discoloration by acid or radicals, formed during IR-light exposure.
The heat-sensitive lithographic printing plate precursors of EP 925916 and EP 1428676 use an IR dye which act for the conversion of IR-radiation to heat and which change color due to the IR-radiation. In these prior art materials, the IR dyes exhibit, beside strong absorption in the IR wavelength range, also side-absorption in the visible wavelength range. Due to IR-exposure, the IR dye decomposes and a print-out image is built-up by the reduction of this side-absorption in the visible wavelength range. A problem of these prior art materials is the low contrast of the print-out images.